From everything I read, despite minor spats between specific individuals now and then, most of the Sede groups of any relevance do have clergy that constantly mingle and cooperate with one another, except, as you pointed out, the SSPV who seem the most insular and absurd on their condemnations of others.

Right, and there has to be some ulterior motive there, since the "reasons" they adduce for their attitude are just utter nonsense.  With the CMRI, for instance, their allegation that they're Old Cathlic schismatics is nonsense.  There's nothing in Canon Law or in any Traditional theology to indicate that you become an Old Catholic by being ordained by one.  I could go right now to an Orthodox bishop, get ordained, and I'd be SUSPENDED ... but it's easy to argue that even suspension is lifted given the necessity created by this crisis, but I would not be considered a schismatic for getting that ordination.  Of course SSPV also ignore the CMRI claim that they asked Brown to recant his errors, and that he did so, before ordaining / consecrating Schuckhardt.  Nor will any of them tell you that Brown was not an Old Catholic but an Old Roman Catholic.  Old Roman Catholics split from the Old Catholics after the latter rejected papal infallibility and adhere to various other heresies, and renounced those heresies.  Theirs was more of a contention that there had been an old agreement in place with Rome to allow the Bishop of Utrecht to have a certain amount of (practical) autonomy from Rome.

After that, then, the argument that the +Thuc line are doubtful also has zero basis in Canon Law or in fact.  But, let's say there was some validity to that arguement.

Uhm, well, since you claim to have valid orders, FIX IT.  They could easily have just gone and conditionally consecrated one another and then have each bishop conditionally ordain/consecrate the others in their group.  Problem solved.  If I for one second thought I had valid orders but some other reasonably-legitimate Catholic group did not, I'd immediately offer to remedy the situation.  Even if the other group did not want to admit any doubt, you could make a case for reasons of peace among the faithful and appeasing their consciences in the absence of a final arbiter of validity.  To me, the thought that so many lay Catholics might be receiving invalid Sacraments and possibly losing their souls ... it's an unspeakable tragedy, and not something to almost gloat about and enjoy having as a weapon to attack the other group(s) with.  So the fact that they behave this way makes me question whether 1) they actually have the faith or 2) they really believe the stuff they're spewing.

No "sede" group and no "R&R" group ARE, as group, THE true Catholic Church, but they may all be Catholic and IN the Catholic Church.  

Careful Laddie boy, what about those pesky material heretics eh? 

During the Great Western Schism, which group was THE true Catholic Church?  Answer is that neither was THE Church, but that both were PART OF the Church, even if they were divided materially regarding the consideration of fact, i.e. who had been legitimately elected pope.

This is a bad comparison, none of those then were divided on doctrine as the various sects are now. Even St. Vincent innocently erred for a time about who was the legitimate pope, but he proved his extraordinary mission with great miracles and was welcomed wherever he went. That material schism is nothing compared to what we are dealing with now - heresy, apostasy.

This is a bad comparison, none of those then were divided on doctrine as the various sects are now. Even St. Vincent innocently erred for a time about who was the legitimate pope, but he proved his extraordinary mission with great miracles and was welcomed wherever he went. That material schism is nothing compared to what we are dealing with now - heresy, apostasy.

Sedevacantist groups really are not too divided on doctrine. The biggest doctrinal divide the Cassiciacuм thesis but it is more a difference in how they view the current papal crisis. Bishop Sanborn of the Roman Catholic institute has a lot of online content but, to simplify the thesis, it is the belief that since John XXIII and his successors were never declared heretics by the church, their elections were valid, but that since they intent to - through Vatican ii - impose a false religion upon the Catholic church, that they never fully accepted papacy and the promise of maintaining the faith that comes with that. 
As you may see it said, they believe they are popes materially, but not formally. 
This is held by the IMBC and bishop Sanborn and his clergy. Other sv groups hold that the papacy is totally vacant. 
If you get to their core, most sede groups teach the exact same things. For instance, the CMRI and the SSPV, the two ends of the sede spectrum one could say, hold virtually the exact same views, the disagreement (which puts them on no speaking terms) is over wether thuc clergy are valid.